Keysight Technologies FieldFox B Series User Manual page 701

Figure 29-1 Marker 2 is Active, marker 1 is NOT active.
About Delta Markers
A Delta marker provides X-and Y-axis readout information that is relative to a 'reference' marker.
A Delta marker is created with a Δ (delta symbol) to distinguish it from the reference marker.
All six markers can be a Delta marker pair.
Peak search functions can be performed using delta markers.
Figure 29-2 A Delta marker and its associated reference marker.
In the graphic above, the marker readout shows the difference between the two markers in
frequency and amplitude.
Keysight N9938-90006 User's Guide
Data Analysis Features
All about Markers
701